Whilst it’s true that the processor is perfectly adequate for basic use on the DS224+ and the DS423+ (my first ‘always on’ Roon Server, used for about 2 months, was hosted on a DS1019+ which used a 4-core Celeron J3455 which is even slower), I think that the more worrying aspect is the 2Gbyte initial installed memory and the 6GBytes that is the maximum supported RAM on these machines.
For small libraries, 6GByte may well be enough on ROCK or Nucleus devices with a stripped down OS running nothing but Roon Server (and the various support services like SMB server and client). However, I’m not so confident that it is sufficient for hosting Roon Server on a NAS which runs a richer set of services and, sooner or later, will likely be asked to do other things as well run Roon Server.
Having said that, since I posted, the OP has stated that he indents to run a library of about 12,000 local tracks. Assuming, he is not integrating Tidal/Qobuz libraries as well, this is not a large library by any means and so he may get away with it. However, if I was looking for a NAS to host Roon, I, personally, would still opt for a device that offers the possibility of upgrading the RAM beyond 8GBytes.
Although I don’t run ROCK, I do something similar. I use a NUC running Roon Server on DietPi with local library storage on the NUC. Every night the database is backed up to my NAS using the Roon Server facility and, additionally, the local library files are sync’d (two way) with a folder on my NAS.
I originally put the local library storage on my Roon Server NUC in order to minimize network traffic when streaming local library content because, at the time, my network was not ideal by any stretch of the imagination. Whilst this is not strictly necessary any more since my network has been significantly upgraded (see the description of the changes to my network in my post at Network Upgrade Journey ), it never hurts to reduce network traffic - especially as some of the other activities I perform are very demanding on the network.